The invention relates to a linear actuator comprising
a first member provided with threads cut in a first direction;
a second member provided to be axially displaceable in relation to the first member and being provided with threads cut in a second direction opposite said first direction;
an intermediary member being engaged with and axially displaceable in relation to the first and second members, respectively, via first and second threads that are complementary to the threads provided on the first and second members, respectively;
means for preventing rotation of the first member in relation to the second member; and
drive means for rotationally driving the intermediary member.
The invention also relates to a medical delivery device comprising such a linear actuator provided as a piston rod.
A linear actuator of this type in the form of a telescopic piston rod is known from the medical delivery device shown in WO 98/47552 that discloses a propelling device for a piston in a container containing a liquid medicament. In one embodiment of the device, the first member constitutes a housing and the second member and the intermediary member extend telescopically from the housing. The intermediary member has the form of a tube provided with an external thread cut in a first direction over its entire length. The thread is complementary to a thread cut in the housing. A gear wheel is rotatably connected to the housing and is inrotatably but longitudinally slidably connected to the intermediary member. This means that when the gear wheel is rotated in relation to the housing, the intermediary member is forced to rotate which, in turn, displaces it out of or into the housing due to the complementary threads cut in the housing and on the outer surface of the intermediary member. The second member is positioned within the intermediary member and it also has the shape of a tube with an external thread over its entire length. This external thread is complementary to an internal thread at the distal end of the intermediary member. These threads are cut in the opposite direction of the external thread of the intermediary member. A guiding rod is provided inrotatably but slidably within the housing and it extends through the intermediary member and is inrotatably but slidably engaged with the interior of the second member. Thus, the guiding rod prevents the housing and the second member from mutual rotation.
Telescopic displacement of the linear actuator is achieved by rotation of the gear wheel which, in turn, forces the intermediary member to rotate and thereby to be displaced out of the housing due to the interaction between its external thread and the thread provided in the housing. The rotation of the intermediary member furthermore displaces the second member out of the intermediary member since the second member is inrotatably arranged in relation to the housing and since the external thread of the second member is opposite the external thread provided on the intermediary member. In this way rotation of the gear wheel causes a telescopic displacement of the intermediary member, the rotation of which, in turn, causes a further telescopic displacement of the second member.
In the linear actuator provided as a telescopic piston rod described above, the exterior surfaces of the intermediary member as well as the second member are provided with threads. In applications where it would be desirable to provide the exterior surfaces of the piston rod with a kind of information, such as a colouring pattern, a scale, a name, etc., this is hardly accomplishable due to the uneven and threaded surfaces. If information is applied to the threaded surfaces it will most likely be worn off over time due to the friction between the members of the linear actuator.
It is therefore an object of the invention to provide a linear actuator of the type mentioned in the opening paragraph that is provided with smooth exterior surfaces that are suitable for incorporation and display of different kinds of information.
This object is achieved by arranging the linear actuator mentioned in the opening paragraph such
that the threads provided on the first and second members are provided as internal threads;
that the threads provided on the intermediary member are provided as external threads;
that the means for preventing rotation of the first member in relation to the second member comprises an outer sheath stretching across the intermediary member, said sheath being inrotatably but longitudinally slidably arranged in relation to the first and second members; and
that the drive means comprises a driving rod that is inrotatably but longitudinally slidably arranged within the intermediary member.
Thereby is obtained that no threads are positioned on the exterior surfaces of neither the first member nor the second member nor on the sheath stretching across the intermediary member. Each of these surfaces may therefore have any desired form and smoothness that allow any kind of information to be displayed thereon. The smooth surfaces also calls for easier cleaning than the threaded surfaces of the prior art linear actuators of the type in question.
Preferably, the sheath is cylinder-shaped with a closed cylinder face with ample opportunities for displaying information thereon.
In first and second embodiments for a linear actuator according to the invention the first and second members are tube-shaped with smooth exterior surfaces that also give ample opportunities for displaying information thereon.
Preferably, the first member has an inner diameter that exceeds an outer diameter of the sheath, and preferably the sheath has an inner diameter that exceeds an outer diameter of the second member, while the intermediary member generally has a smaller diameter than an inner diameter of the second member, except for a part at one end that is provided with the external threads that engage the internal threads of the first member. This structure of the linear actuator gives a true telescopic appearance of the linear actuator, exposing only smooth surfaces. The smooth surfaces makes it possible to seal the exposed parts in relation to each other, e.g. by means of O-rings.
In the first embodiment of the invention ordinary threads are employed, but in the second embodiment of the invention the external threads provided on the intermediary member are formed by a plurality of balls that protrude from a ball track in a re-circulating ball arrangement. By this arrangement the friction between the intermediary member and the first and second members is reduced considerably, which increases the efficiency of the threaded connection.
In order to prevent the first and second members from rotating relative to each other the sheath is preferably provided with at least one external tongue that engages an internal longitudinal groove provided in the first member, while the second member is provided with at least one external tongue that engages an internal longitudinal groove provided in the sheath.
In a third embodiment of a linear actuator according to the invention the intermediary member is provided with crossing threads, while each of the first member and the second member has at least one leg, which legs complement each other circumferentially around the intermediary member. In this embodiment the overall diameter of the linear actuator can be less than the overall diameter in the first and second embodiments, since the diameter of the threaded parts can be maintained at a level corresponding to the smallest diameter in the first and second embodiments. A reduction of the diameter entails an increase of the angle of pitch, which in turn increases the efficiency of the threaded connection.
Each of the first and second members preferably has two legs, each with a circumferential expanse of less than 90xc2x0. Providing only two legs at each member simplifies the structure thereof and gives opportunities to provide relatively large surfaces for displaying information.
In this embodiment the sheath has an inner diameter that exceeds the outer diameters of the first and second members, while it is provided with internal grooves that engage the legs of the first and second members. In this embodiment the sheath with ample opportunities for being provided with information is fully visible at all times since it covers the first as well as the second member.
When the linear actuator according to the invention is employed as a piston rod in a medical delivery device provided with a cartridge with a liquid medicament, it is preferably arranged in such a way that neither the second member nor the sheath has an outer dimension that exceeds an inner diameter of the cartridge. Thereby the second member as well as the sheath of the linear actuator can be expanded into the cartridge for expelling the liquid medicament therefrom.
In a preferred embodiment of the medical delivery device the driving rod is provided with a gear wheel that is connected to a driving mechanism, such as an electric motor.